Plate-making has undergone a marked change from manual working to electronic stripping during the last few years. Along with such trends, use of plotters such as an image setter are rapidly spreading. A processor of conventional silver salt photographic materials is now commonly connected on line to such precision instruments, producing problems such a corrosion of the substrate or troubles of expensive instruments, which increasingly occur due to gas or moisture released from the processing solutions in the processor.
In conventional silver salt photographic materials, such works that plumbing for diluting the developer and fixer, as well as for washing is needed and effluents which have to be recovered by recyclers take a lot of time and labor, so that introduction of a water-free dry processing system is strongly desired. Among current dry systems, thermal processing using thermally developable photothermographic materials is currently most suitable for practical use in terms of manufacturing cost and performance.
However, the photothermographic materials are often processed at a temperature higher than the glass transition temperature of the support so that the photothermographic materials are often deformed due to elongation or shrinkage after being processed, producing problems such that images on the photothermographic material are not reproduced in the intended dimension due to elongation or shrinkage of the support. Accordingly, when applied to color printing, difference in dimension between separation negatives (or positives) occurs, producing doubling on prints.
Attempts for improving dimensional stability have been proposed. JP-A 61-235608 and 3-275332 (herein, the term, JP-A means an unexamined and published Japanese Patent Application), for example, describes a method of relaxation after thermal fixing during the stage of casting of the base substrate. The support is often subbed, but subbing adversely enhances thermal shrinkage so that a more reliable method is eagerly sought. JP-A 10-10676 and 10-10677 describe a thermal treatment after casting of the base substrate. However, its reproducibility was insufficient and when applied to color printing, it still causes doubling due to differences in dimension between separation negatives and is therefore unacceptable in practice.
Further, thermal processing of photothermographic materials results in other problems. A thermal developing section often produces temperature unevenness in its interior. On the other hand, the photothermographic material easily causes uneven development even when the temperature difference is only .+-.1.degree. C. Accordingly, image sizes or halftone dot sizes vary locally with temperature unevenness inside the thermal developing section, leading to uneven development. As a result, suitable images cannot often be reliably obtained. As a result of the inventor's study, it was proved that problems concerning the image size were often produced when photothermographic materials were subjected to thermal development. Such problems concerning the image size cause doubling in the field of printing, in which plural photographic materials are overlapped, leading serious problems.